The solvent-driven formation of multi-morphological Ag-CeO2 plasmonic photocatalysts with enhanced visible-light photocatalytic reduction of CO2

Article abstract of DOI:10.1039/c8ra08938h

Ag-CeO₂ plasmonic photocatalysts with multiple morphologies were synthesized via a simple solvent-driven method. The phase compositions, morphologies and optical properties of the samples were systematically investigated. A combination of noble metal Ag and semiconductor CeO₂ in certain solvents (such as methanol and ethylene glycol) enhanced surface plasmon resonance (SPR), which was attributed to the good dispersion of Ag particles on CeO₂ and high Ag⁰ ratios on the surface. The enhanced SPR effect boosted absorption of incident light and facilitated charge carrier separation and transport efficiency caused by the formation of Schottky barriers, thus promoting VLPCR performance. The optimum ACG sample (ethylene glycol was adopted as the solvent) exhibited the maximum VLPCR activity, achieving a CH₄ yield of 100 μmol and a CH₃OH yield of 35 μmol per gram of catalyst per hour during 6 h visible-light irradiation.

Full text of DOI:10.1039/c8ra08938h

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PAPER
The solvent-driven formation of multi-
morphological Ag–CeO2 plasmonic photocatalysts
with enhanced visible-light photocatalytic
reduction of CO2
Cite this: RSC Adv., 2018, 8, 40731
a
Yunpeng Shi,a Yunxia Zhao,a Mindong Chen,a Qin Zhongb
*
Wei Cai,
a
and Yunfei Bu
Ag–CeO2 plasmonic photocatalysts with multiple morphologies were synthesized via a simple solvent-
driven method. The phase compositions, morphologies and optical properties of the samples were
systematically investigated. A combination of noble metal Ag and semiconductor CeO2 in certain
solvents (such as methanol and ethylene glycol) enhanced surface plasmon resonance (SPR), which was
attributed to the good dispersion of Ag particles on CeO2 and high Ag0 ratios on the surface. The
enhanced SPR effect boosted absorption of incident light and facilitated charge carrier separation and
transport efficiency caused by the formation of Schottky barriers, thus promoting VLPCR performance.
The optimum ACG sample (ethylene glycol was adopted as the solvent) exhibited the maximum VLPCR
activity, achieving a CH4 yield of 100 mmol and a CH3OH yield of 35 mmol per gram of catalyst per hour
during 6 h visible-light irradiation.
Received 29th October 2018
Accepted 27th November 2018
DOI: 10.1039/c8ra08938h
rsc.li/rsc-advances
photocatalysis has been proposed, since the investigation of
it in Awazu's work.18 Specically, when the wavelength of the
1. Introduction
incident light matches the oscillation frequency of free
electrons on plasmonic metals, then a SPR effect will be
induced, thus enhancing absorption of the incident light.
Moreover, the combination of a noble metal and semi-
conductor can heighten electron–hole separation efficiency,
via the formation of Schottky barriers.19 Among the noble
metal-based photocatalysts, the relatively inexpensive Ag-
based photocatalysts have been investigated the most. Yu
et al. prepared Ag/TiO2 via a simple silver mirror reaction for
VLPCR, and the results showed that the SPR, which enhanced
photocatalytic activity, was closely related to the size and
loading amount of Ag.20 Gui et al. synthesized Ag-
MWCNT@TiO2 (multi-walled carbon nanotubes@titanium
dioxide) core–shell nanocomposites via a one-pot technique,
and they found that a 2 wt% Ag loading exhibited excellent
VLPCR performance, which was 6.34 mmol gꢀ1-catalyst for
CH4 and 0.68 mmol gꢀ1-catalyst for ethylene.21 Hetero-
structured g-C3N4/Ag–TiO2 hybrid photocatalysts were re-
ported in Li's work, and the optimal catalyst with a g-C3N4 to
AgTi mass ratio of 8% displayed the maximum VLPCR
activity, obtaining a CH4 yield of 28 mmol and CO yield of 19
mmol per gram of catalyst.22
Currently, the energy shortage and greenhouse effect caused
by excessive carbon dioxide (CO2) emissions are realized as
major serious problems.1–3 Among plentiful solutions, the
direct visible-light photocatalytic reduction of CO2 (VLPCR)
into chemical fuels such as methane (CH4), methanol
(CH3OH) and formic acid (HCOOH) is considered as a most
promising strategy.4–6 Aiming to promote industrial applica-
tions of VLPCR, a key challenge is exploring for suitable pho-
tocatalysts with high stability and photocatalytic efficiency.7,8
Until now, quite a number of photocatalysts have been inves-
tigated as candidates for VLPCR including TiO2, CdS, BiVO4, g-
C3N4, et al.9–12 However, problems such as low incident visible-
light utilization and fast electron–hole recombination still
need to be improved.13–15
Among the abundant improvement methods, the surface
plasmon resonance (SPR) effect between noble metal nano-
particles and semiconductor photocatalysts recently has
aroused broad attention due to its boosted enhancement of
photocatalysis.16,17 Therefore, the concept of plasmonic
aJiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment
Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment
Monitoring and Pollution Control, School of Environmental Science and
In order to enhance VLPCR performance through a SPR
effect, the physico-chemical structure of the semiconductors is
another crucial aspect that needed to be considered. Among
the common semiconductors, CeO2 has been thoroughly
Engineering, Nanjing University of Information Science
& Technology, Nanjing,
210044, P. R. China. E-mail: caiwei_19880105@163.com
bSchool of Chemical and Engineering, Nanjing University of Science and Technology,
Nanjing, 210094, P. R. China
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investigated for various heterogeneous catalytic reactions, methanol, ethanol, isopropanol, and ethylene glycol,
including de-NOx, water-gas shi reactions, CO oxidation, and respectively.
photocatalysis23–26 owing to its abundance of surface oxygen
defects and convenient morphology control. Microstructured-
CeO2 or its composites (e.g., Pd/CeO2) with various morphol-
ogies such as spherical,27 cubic,28 ower-like,29 and coralloid30
morphology have been reported. Li et al. synthesized meso-
structured-CeO2/g-C3N4 nanocomposites for a VLPCR test, and
the maximum CO and CH4 yield reached up to 11.8 mmol and
13.88 mmol per gram of catalyst, respectively.31 They conrmed
that the photo-generated electrons were trapped by Ce4+ in the
mesostructured-CeO2, leading to recombination prevention of
photogenerated electrons and holes, and thus enhancing the
VLPCR performance.
2.2 Characterizations
Powder X-ray diffraction (XRD) patterns of the samples
prepared in different solvents were recorded in the 10–80ꢁ 2q
range (scan speed ¼ 8ꢁ$minꢀ1, step value ¼ 0.04ꢁ) using an XD-
3 diffractometer (Beijing Purkinje General Instrument Co.,
˚
Ltd., China) (Cu Ka radiation, l ¼ 1.5418 A).
Raman spectra were recorded on a Renishaw Invia Raman
Microscope with Ar + radiation (514 nm). The laser light was
focused on the samples by using a microscope equipped with
a 6100-objective lens.
The BET surface areas of all samples were obtained on an
Autosorb-iQ Analyzer (Quantachrome Instruments). The pore
structures were analyzed with the Barrett–Joyner–Halenda (BJH)
method. Before the characterization, the samples were dried at
Up to now, investigations of semiconductors with Ag-
containing plasmonic photocatalysts mainly focused on
TiO2, g-C3N4, etc. but work on Ag–CeO2 plasmonic compos-
ites for VLPCR still has not been reported. Herein, in this
work, Ag–CeO2 plasmonic photocatalysts with multi-
morphologies were synthesized via a simple solvent-driven
method, and ve common solvents (H2O, methanol,
ethanol, isopropanol, and ethylene glycol) were used. The
best VLPCR performance was observed over an ACG sample
(ethylene glycol was used as the solvent), which obtained
a CH4 yield of 100 mmol gꢀ1 hꢀ1 (mmol per gram of catalyst per
ꢁ
180 C for 12 h under vacuum.
Field-emission scanning electron microscope (SEM) obser-
vations were carried out using a Quanta 250F (FEI, U.S.A.).
High-resolution transmission electron microscopy (HR-
TEM) studies were performed on a JEM-2100 electron micro-
scope (JEOL, Japan) with an accelerating voltage of 200 kV that
was equipped with a slow-scan charge-coupled device (CCD)
camera.
X-ray photoelectron spectroscopy (XPS) analyses were per-
formed on an ESCALAB 250 spectrometer (Thermo, U.S.A.). The
X-ray source was Al Ka X-ray radiation (hn ¼ 1486.6 eV). The
analyses were performed at room temperature, and the pres-
sures were typically on the order of less than 10ꢀ8 Pa. The
reaction time (hour)) and CH3OH yield of 35 mmol gꢀ1 hꢀ1
.
This satisfactory photocatalytic activity was deduced to be
caused by an enhanced SPR effect, and the specic enhanced
reason was investigated by characterizations using XRD,
Raman, XPS, UV-Vis, PL, and photoelectrochemical
measurements. An enhancement mechanism was also
proposed based on the experimental results. The ultimate
goal was to explore a photocatalyst with more efficient
conversion of CO2 into chemical fuels.
ꢁ
samples were outgassed overnight in a vacuum oven at 10 C
before acquiring the XPS measurements. All binding energies
(BE) were referenced to the adventitious C 1s peak at 284.4 eV.
The penetration depth of the XPS probe is 10 nm.
Diffuse reectance spectroscopy (DRS) was carried out on
a Shimadzu UV-2550 UV-Vis spectrophotometer (Shimadzu,
Japan). BaSO4 was the reference sample, and the spectra were
recorded in the range of 220–1000 nm.
2. Experimental
2.1 Preparation of the catalysts
Photoluminescence spectra (PL) were obtained with an
ELabram-HR800. The excitation wavelength was set at 325 nm,
and the spectra were scanned in a range of 350–600 nm.
All starting reagents were purchased from Alfa Aesar (Thermo
Fisher Scientic, China) and used without further purica-
tion. Deionized water aer a Milipore system puried it was
used throughout all experiments. AgNO3, Ce(NO3)3$6H2O,
and urea were dissolved in 60 mL of the solvent (H2O,
2.3 Photoelectrochemical measurements
methanol, ethanol, isopropanol, and ethylene glycol) under Photoelectrochemical measurements were performed on a CHI
magnetic stirring. Subsequently, the mixed solution was 760B electrochemical workstation (CH Instruments Ins., China)
transferred into Teon containers (100 mL), sealed in an with a conventional three-electrode system in which a Pt wire
autoclave, and hydrothermally reacted at 180 ꢁC for 24 h. and a Ag/AgCl electrode (3 mol Lꢀ1 KCl) served as the counter
Aer washing and centrifugation with deionized water and and reference electrodes, respectively. A 0.5 mol Lꢀ1 Na2SO4
ethanol several times, the mixture was dried at 60 ꢁC in aqueous solution was used as the electrolyte. The photo-
ꢁ
a vacuum oven overnight and calcinated at 500 C for 4 h in electrodes were prepared by a drop casting method. Ten milli-
a pure He atmosphere, following which the nal catalysts grams of photocatalyst powder was dispersed in 1 mL of ethanol
were obtained. The mass ratio of Ag to Ce over all catalysts containing 10 mL of naphthol to form a homogeneous suspen-
was set as 5 : 1, and the mole ratio of urea to the total metallic sion by an ultrasonic method. All samples were deposited onto
element content (including Ag and Ce) was assigned as 3. The Fluorine-doped Tin Oxide (FTO) as a thin lm, and the coated
synthesized samples were denoted as ACH, ACM, ACE, ACI, area was xed at 1.5 cm2. Aer being dried at 180 ꢁC for 12 h in
and ACG separately, representing the solvents of H2O, a vacuum oven, a homogeneous lm was obtained.
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These results indicated that the application of H2O as the
solvent did not benet for the formation of homogeneous Ag–
CeO2 composites. The same situation also occurred with the
ACI sample, in which isopropanol was used for the preparation.
As for ACM, ACE and ACG samples, no distinct peaks assigned
to Ag0 or other Ag-contained compounds were found, and no
obvious shis were found when compared to the standard cubic
CeO2, thus displaying a good dispersion of Ag species over
CeO2. Moreover, the four main peaks over ACM and ACG
samples were broadened to a certain degree when compared to
the ACE sample. This indicated that particles sizes of the ACM
and ACG samples were smaller than those of the ACE sample,
based on the Scherrer formula. Hence, it was concluded from
XRD results that use of the solvents during the synthetic process
played a key role with the crystal structures of the Ag–CeO2
composites.
The SPR effect of Ag particles over CeO2 was investigated by
Raman spectra, as shown in Fig. 1(b). For all the samples, only
one Raman band at 445 cmꢀ1 was found in the range from
200 cmꢀ1 to 1000 cmꢀ1, which was assigned to the F2g vibration
mode of cubic CeO2.32 Besides, no peak position shi was
observed in the Raman spectra among these samples, indi-
cating that the main structures of these samples were the same.
However, an apparent increase in intensity of Raman peaks was
observed, which was due to the surface enhanced Raman scat-
tering effect (SERS).33 This meant that the electrons on the
surface of Ag particles were photoexcited to a high energy state
by the Ar+ laser during the Raman spectroscopy experiment.
These high energy state electrons could induce a strong near
eld on the Ag particle surfaces. The Raman peaks over ACM
and ACG samples were more intense than in other samples,
indicating that the SERS and SPR effects of the surface Ag
particles were enhanced over the ACM and ACG samples.
The specic surface areas and the pore structures of all
samples were characterized as shown in Fig. 1(c) and (d). All
samples displayed isotherms of type IV, in which ACH, ACM,
and ACG exhibited hysteresis loops of type H3, while the rest of
the samples showed hysteresis loops of type H4.34 The H3 type
indicated the existence of slit-like mesopores, while the H4 type
represented cranny-like pores between mesopores and micro-
pores.35 The pore distributions of ACH and ACI samples located
in a higher range, compared to other samples, are shown in
Fig. 1(d).
2.4 Photoreduction of CO2
A CO2 photocatalytic reduction was typically conducted in a gas-
closed circulation system using a 200 mL reactor (Anhui Kemi
Machinery Technology Co., Ltd., China). The reactor was made
of stainless steel, and the visible light passes through a sapphire
at the top. In a typical process, 50 mg photocatalyst was
uniformly dispersed into the black Teon container with 15 mL
deionized water, and the suspension was kept stirring. In order
to ensure that air was eliminated, the reactor was rst vacuu-
mized to the vacuum degree of 0.08 MPa, then the reactor was
infused with high purity CO2 (99.99%) gas to a pressure of
0.6 MPa. Next the system was vacuumized again. A washing
cyclic process was conducted for at least ve times, and nally
the reaction system was sealed and maintained at 0.6 MPa for
the following CO2 photoreduction. A 300 W Xe lamp (CEL-
HXF300, Beijing CEAulight Co., Ltd., China) with a UV-light
cutoff lter (l > 420 nm) was used as the light source. The
temperature of the reactor was controlled at 60 ꢁC via an electric
jacket. Aer photoreaction for 6 h, the product in the reactor
was slowly injected into a gas chromatograph (GC-9860, Nanj-
ing Hope Corp., China). The reaction products such as CH4,
CH3OH, or other carbon-based gases were analysed via a ame
ionization detector (FID) equipped with a 5A molecular column
and a TDX-01 packed column.
3. Results and discussion
3.1 Crystal structural and morphology characterization
Wide angle XRD patterns were conducted to analyse crystal
structures of the Ag–CeO2 composites, as shown in Fig. 1(a).
Except for ACH, the rest of the samples displayed four main
peaks at about 28.6ꢁ, 33.1ꢁ, 47.5ꢁ, and 56.3ꢁ, which are in accord
with the corresponding (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes
of the cubic CeO2 (JCPDS no. 34-0394).27 In the ACH sample,
besides the peaks assigned to the cubic CeO2, other peaks also
existed. Peaks at about 38.1ꢁ and 44.0ꢁ were ascribed to
elemental silver, while the peaks at about 17.7ꢁ, 24.6ꢁ, 30.5ꢁ,
and 43.5ꢁ were assigned to the unreacted AgNO3 precursor.
It was then deduced that the average pore sizes of these ACH
and ACI samples were higher than the rest of the samples.
Specic BET surface areas and pore structure parameters are
Table 1 Surface areas and pore structures of Ag–CeO2 composites
with different solvents
BET surface
area (m2 gꢀ1
Pore volume
Pore size
(nm)
Sample
)
(mm3 gꢀ1
)
ACH
ACM
ACE
ACI
8.9
48.8
39.3
30.9
52.1
30.1
27.8
56.8
100.9
46.5
31.6
6.1
6.0
31.8
4.8
Fig. 1 XRD patterns (a), Raman spectra (b), N2 adsorption–desorption
isotherms (c), and pore size distributions (d) of Ag–CeO2 composites
with different solvents.
ACG
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Fig. 2 SEM images (a–e) of Ag–CeO2 composites with different
solvents. (a) ACH, (b) ACM, (c) ACE, (d) ACI, and (e) ACG. Energy
dispersive spectroscopy (EDS) (f) results of the ACG sample.
listed in Table 1. The deductions concerning the pore sizes of all
samples were conrmed, and it was found that ACG displayed
the maximum BET surface area and the minimum pore size.
These high surface areas could promote adsorption of the
reactant gas, thus enhancing the photocatalytic conversion.
SEM characterization was adopted to view the morphologies
of all samples at the micrometre scale, as shown in Fig. 2, where
it can be seen that using different solvents during preparation
resulted in a distinct inuence on the morphologies of the
samples. The morphologies of ACH, ACM, ACE, ACI, and ACG
exhibited octahedral, fusiform, ruleless, spherical, and sheet-
like shapes, respectively. Among all samples, the morphology
of ACM and ACG was uniform, while the rest were irregular and
nonuniform, such as the spherical particle's sizes in the ACI
sample. In addition, the morphologies obtained from SEM were
in accord with the BET analysis. The elemental concentrations
in the ACG sample were characterized via EDS (Energy Disper-
sive Spectrometer), and the results showed that the mole ratio
of Ag to Ce was about 1 : 10, which was close to the theoretical
value of the precursor dosage. This indicated that the Ag and Ce
precursors all participated the forming process of the ACG
sample, and this process could benet the uniformity in the
morphology.
Fig. 3 TEM images (a), HR-TEM images (b), selected-area electron
diffraction (SAED) (c), and the element mapping images (d) of ACG
samples.
dispersion of Ag0 was expected to enhance the SPR effect via
providing more SPR spots for the photocatalytic reaction to
occur.33
3.2 Photoreduction of CO2 performance
The catalytic performance of photocatalysis is crucial from the
point of practical applications.36 Therefore, all samples were
evaluated via the VLPCR test, as shown in Fig. 4. It can be seen
that the CH4 yield rates over all samples were all higher than the
CH3OH yield rates. Especially, there was no CH3OH evolution
over the ACH sample. Among all samples, ACM and ACG
samples displayed relatively high CH4 and CH3OH yield rates,
which were 89.32 mmol gꢀ1 hꢀ1 and 99.98 mmol gꢀ1 hꢀ1 for CH4,
and 32.13 mmol gꢀ1 hꢀ1 and 34.96 mmol gꢀ1 hꢀ1 for CH3OH,
respectively.
Combined with the crystal structure and morphology anal-
ysis, it was concluded that the high VLPCR performance was
related to the uniform morphology and the good dispersion of
Ag species, which was a key factor in this Ag-based plasmonic
photocatalyst. Selectivities of the corresponding products over
all samples were calculated as shown in Fig. 4(b). The CH4
selectivities of ACE and ACI samples were higher than CH3OH
selectivity, while the opposite result was found over the ACM
sample. The ACG sample displayed almost the same CH4 and
CH3OH selectivity. The CH3OH selectivity of the ACM sample
In order to investigate the existence form of Ag species on
CeO2, TEM and HR-TEM characterizations were adopted to
observe their morphologies at the nanometre scale, as shown in
Fig. 3. At the nanometre scale, it was seen that small particles
existed on CeO2, and they were relatively well dispersed. From
HR-TEM images, it is seen that there were two kinds of inter-
planar spacing.
One of them exhibited a value of 0.316 nm, and the other one
displayed a value of 0.238 nm. The former could be ascribed to
the (1 1 1) plane of CeO2, and the later was assigned to the (1 1 1)
plane of Ag0. The selected area electron diffraction (SAED) of the
ACG sample displayed two continuous rings, which corre-
sponded to the (1 1 1) and (2 0 0) planes of CeO2. The dispersive
diffraction ring was caused by agglomeration of the sheets in
the ACG sample. In addition, no diffraction ring corresponding
to Ag0 could be found, which was due to the good dispersion of
Ag0 and the small particle sizes. EDS element mapping was
adopted to investigate the element distribution of the sheet,
and the results revealed a uniform distribution of each element
on the sheet, which was in accord with XRD results. The good
Fig. 4 Photocatalytic CH4 and CH3OH evolution performance (a) and
the selectivity of the corresponding product over Ag–CeO2
composites with different solvents (b).
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Table 2 Comparison of CH4 and CH3OH evolution amounts for CeO2-based samples
Sample
CH4 yield amount/mmol gꢀ1 hꢀ1
CH3OH yield amount/mmol gꢀ1 hꢀ1
Ag–CeO2 [our work]
CeO2–TiO2 (ref. 37)
Ag/Ag3PO4/CeO2 (ref. 38)
CeO2/Bi2MoO6 (ref. 39)
CeO2–TiO2 (ref. 40)
CeO2/g-C3N4 (ref. 31)
99.98
30.2
N/A
N/A
N/A
34.96
N/A
42.5
58.4
18.6
N/A
13.88
reached up to 55.65%. A comparison of CH4 and CH3OH a low Ag precursor concentration. Fig. 6(b) exhibits a typical XPS
evolution amounts for CeO2-based samples is shown in Table 2. spectrum of Ag 3d, in which peaks at about 374 and 368 eV were
It can be seen that different CeO2-contained composites were attributed Ag 3d3/2 and Ag 3d5/2, respectively. Specically, all
synthesized. However, the products CH4 and CH3OH could not samples were numerically tted into the metallic Ag0 and Ag+
be obtained simultaneously from these samples, and the CH4 states, and for each electron orbit the tted peak at high
yield amounts over certain samples was far lower than the binding energy was assigned to the metallic Ag0 while the peak
values in this work. The CH3OH evolution amount here was at low binding energy belonged to the Ag+ state.41 In addition,
relatively high, although some samples displayed higher values. there was no obvious peak shi among these samples, but the
As a short summary, compared with other CeO2-based photo- surface concentrations of metallic Ag0 were markedly different.
catalysts, the Ag–CeO2 plasmonic photocatalyst prepared via The results are shown in Table 3 where it can be seen that ACH
a simple solvents-driven method in this work displayed exhibited the lowest surface Ag0 content among all samples
delightful CH4 and CH3OH yields caused by the SPR effects.
and, by contrast, ACM and ACG possessed relatively higher Ag0
The stability of semiconductor photocatalytic materials can't content. It was reported that the SPR effect could be enhanced
be neglected. Therefore, four cyclic CH4 and CH3OH evolution by an increase of the surface Ag0 ratios,41 which should be under
tests over Ag–CeO2 plasmonic photocatalyst were carried out, in the premise of a good dispersion of Ag species. Hence, the
which the optimized ACG sample was applied. The stability test surface Ag0 ratios played the key role in the photocatalytic
is described as where a used sample is collected by a centrifugal reaction, and the VLPCR results further conrmed this
process for the next VLPCR test. As observed in Fig. 5, the CH4 deduction.
and CH3OH yields were decreased slightly aer a fourth cyclic
The Ce 3d spectrum was tted into eight peaks in two
test, indicating that the synthesized Ag–CeO2 plasmonic pho- different regions (3d5/2: v series and 3d3/2: u series) via Gaussian
tocatalysts possessed satisfactory stability properties.
tting, as shown in Fig. 6(c), in which the peaks labelled v, v00, v000
and those labelled u, u00, u000 were ascribed to the Ce4+ state,
while the v0 and u0 peaks were assigned to the Ce3+ state.42 For
the O 1s spectra, two main peaks were separated aer tting
and the peak at a lower binding energy was denoted as Oa
corresponding to the lattice oxygen (O2ꢀ), while the peak at
3.3 XPS characterization
In order to investigate enhancement of the SPR effect of Ag
nanoparticles on VLPCR, several kinds of characterizations
were adopted, and XPS analysis was rst applied, as shown in
Fig. 6. The survey spectra displayed in Fig. 6(a) conrmed the
existence of Ag, Ce, and O in all catalysts, but the peak inten-
sities corresponding to Ag 3d were all weak, which was due to
Fig. 6 XPS spectra of Ag–CeO2 composites with different solvents: (a)
Fig. 5 Cyclic stability tests of an ACG sample.
survey; (b) Ag 3d; (c) Ce 3d; and (d) O 1s.
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Table 3 XPS elementary cation surface concentrations of Ag–CeO2
composites with different solvents
square form of Ahv indicated that the band gaps of these
samples were all direct band gaps. Specic Eg value are shown in
Table 4, and among them ACM, ACE, and ACG samples
exhibited a relatively small Eg value. Combined with XRD
analysis, the narrowed band gap was probably due to the good
dispersion of Ag particles over CeO2.22
Cation ratios
Sample
Ag0/Ag(total)
Ce3+/Ce(total)
Ob/(Oa + Ob)
ACH
ACM
ACE
ACI
50.38%
86.11%
66.38%
65.27%
85.32%
17.05%
21.18%
18.27%
19.71%
25.25%
32.91%
90.53%
32.64%
38.92%
95.52%
More information about the photoelectric properties were
explored via PL and photoelectrochemical measurements, as
shown in Fig. 8. Among these characterizations, Mott–Schottky
measurements were conducted over all catalysts to clarify their
band structures. Reversed sigmoidal plots were observed with
an overall shape consistent with that typical for n-type semi-
conductors, as shown in Fig. 8(a). The at band potentials (V)
of all samples were calculated from the x intercept of the linear
region, where the units are V vs. Ag/AgCl. Based on the equation
E (NHE, normal hydrogen electrode) ¼ E(Ag/AgCl) + 0.197 V,44
V was converted into the NHE potential. In general, V is close
to the bottom of the conduction band (ECB) of n-type semi-
conductors, and ECB is more negative than V (here, the value of
0.1 V vs. NHE was adopted for the calculation of ECB over all
samples). Hence, according to the above analysis, the band
structures of all samples are summarized in Table 4. It can be
seen that ACM and ACG exhibited more negative ECB values
than other samples, while ACH possessed the highest ECB value
among all samples. Generally, the catalysts with more negative
ECB values possessed a stronger reducing capacity. Therefore,
ACM and ACG displayed a stronger reducing capacity than other
samples, which was attributed to the enhanced SPR effect
caused by the good dispersion of Ag particles and high Ag0
ratios. Moreover, according to eqn (1) and (2), the samples
synthesized in this work all possessed a reduction potential to
convert CO2 into CH4 and CH3OH, except for ACH. Only CH4
could be generated theoretically over ACH. Hence, no CH3OH
was detected over ACH in the VLPCR test.
ACG
a higher binding energy was nꢀamed Ob corresponding to
chemisorbed oxygen, such as O2 or Oꢀ.43 From the surface
cation concentration results in Table 3, it can be seen that ACM
and ACG displayed higher Ce3+ and Ob ratios than other
samples. This could be ascribed to high Ag0 ratios and the good
dispersion of Ag species on the surface. It is known that Ce3+
and Ce4+ can be interconverted via the transfer of an electron,
and the surface Ag0 could transfer this electron easily, thus
promoting the generation of Ce3+. On one hand the transferred
electron could be reacted with O2 to form chemisorbed O2ꢀ, but
on the other hand the formation of Ce3+ distorted the lattice
and then promoted the generation of Oꢀ. Therefore, the high
Ce3+ and Ob ratios in ACM and ACG samples indicated their
good electron transport abilities, thus leading to the high
VLPCR performance.
3.4 Band structure and photoelectric properties analysis
Since the optical absorption properties of a photocatalyst play
a signicant role in determining photocatalytic performance,
information on the band structure was investigated via UV-Vis
DRS measurements, as shown in Fig. 7. All catalysts exhibited
an absorption band centred at 270 nm, which was ascribed to
the Ce4+ ) O2ꢀ charge transfer. Besides, except for the ACI
sample, the rest of the samples also displayed absorption
caused by the Ce4+ ) O2ꢀ interband, which was located at
360 nm.23 In addition, it can be seen that the light adsorption of
ACM and ACG samples was enhanced both in the UV and visible
light regions, and this was attributed to the SPR effect of the Ag
particles over CeO2,20 in accord with Raman and XPS analysis.
Based on the Kubelka–Munk function, the UV-Vis spectra were
converted into Tauc plots, as shown in Fig. 7(b).
CO2 + 8H+ + 8eꢀ / CH4 + 2H2O E0(V) ¼ ꢀ0.24 eV
(1)
CO2 + 6H+ + 6eꢀ / CH3OH + H2O E0(V) ¼ ꢀ0.38 eV (2)
The separation and transfer of charge carriers in these
samples was investigated via their photoluminescence spectra,
as shown in Fig. 8(b). It is well known that PL emission intensity
can reect the separation and recombination of electrons and
holes on the surface of a photocatalyst. A low emission intensity
conrms a suppressed recombination rate of photoinduced
electrons and holes, and vice versa.45 Among all samples, ACH
The band gap (Eg) was obtained via the point of intersection
between the tangent line of the Tauc plot and the y ¼ 0 plot. The
Table 4 The band structure parameters of Ag–CeO2 composites with
different solvents
Sample
ECB (eV)
EVB (eV)
Eg (eV)
ACH
ACM
ACE
ACI
ꢀ0.31
ꢀ0.61
ꢀ0.49
ꢀ0.43
ꢀ0.67
2.70
2.19
2.37
2.56
2.02
3.01
2.80
2.86
2.99
2.69
Fig. 7 UV-Vis diffuse reflection spectra (a) and converted Tauc plots of
Ag–CeO2 composites with different solvents (b).
ACG
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Fig. 9 (a) Schematic illustration of the band structure of Ag and CeO2
before and after contact. (b) Schematic illustration for SPR induced
“hot-electron” migration on the Ag–CeO2 composite. (c) Schematic
diagram of band structure and redox potential of Ag–CeO2
composites with different solvents.
Fig. 8 Mott–Schottky plots (a), photoluminescence (PL) spectra (b),
EIS Nyquist plots (c), and photocurrent potential curves (d) of the Ag–
CeO2 composites with different solvents.
addition, aer contact, a positive space-charge region was
formed on the surface of the semiconductor, which resulted in
the electron energy of the surface being higher than that in the
bulk. Therefore, the energy band was bent upward and
a Schottky barrier was generated, which prevented the electrons
in the semiconductor from transferring to metals.
exhibited the strongest PL emission peak intensity at 400 nm,
which belongs to the radiative recombination of photoinduced
electrons and holes on the surface of ACH. Compared to ACM,
ACE, and ACG, these three samples displayed weaker peak
intensity, corresponding to the larger separation ability of
photogenerated electrons and holes.
However, under light irradiation, the highly energetic “hot-
electrons” generated by the SPR effect of the Ag particles could
overcome the Schottky barrier between CeO2 and Ag particles
and migrate to the neighbouring CeO2, leading to the increase
in surface electron density (Fig. 9(b)). These photoexcited hot
electrons could be utilized during the VLPCR test. In addition,
the near eld effect induced by the Ag particles could accelerate
the transport of electrons on CeO2, thus improving the charge
carrier transport efficiency. The good dispersion of Ag particles
over CeO2 could create multiple SPR active sites, which was
benecial for further intensifying the near eld effect to
enhance charge carrier transport efficiency. Although VLPCR
performance could be enhanced by the SPR effect, not all
samples possessed the reduction potential to generate the
production of CH3OH, such as ACH, shown in Fig. 9(c). In
general, catalysts with more negative ECB values possessed
As a matter of fact, the efficient separation of photoinduced
electrons and holes also could be reected in the much smaller
diameter of the semi-circular Nyquist curve of electrochemical
impedance spectroscopy (EIS) and the increased photocurrent
density under visible light.46 A small radius usually indicates
a low charge transfer resistance and a high separation efficiency
of photogenerated electron–hole pairs.47 As shown in Fig. 8(c),
ACM and ACG displayed a smaller radius among all samples,
indicating that these two samples possessed higher separation
efficiency, which was in accord with PL analysis. A similar
phenomenon was observed in the photocurrent test, as shown
in Fig. 8(d), where ACG displayed the largest photocurrent
density during on–off cycles of intermittent visible light irradi-
ation among all samples, and ACM took second place. The
improved separation and transfer efficiency of charge carriers
over ACM and ACG was due to the enhanced SPR effect, which
was induced by the good dispersion of Ag particles and high Ag0
ratios. Therefore, it can be concluded from UV-Vis, PL, and
photoelectric test results that the enhanced SPR effect could
lead to the negative ECB value, the narrowed band gap and the
excellent photoelectric properties, and these features played
a signicant role in the photocatalytic reaction, thus improving
the VLPCR performance.
a
stronger reducing capacity. Among all samples, ACG
possessed the most negative ECB values, and coupled with the
strong SPR effect caused by the uniform distribution of Ag
particles and the highest Ag0 ratios, thus the largest CH4 and
CH3OH evolution amount was obtained over ACG.
4. Conclusions
In summary, a simple solvent-driven method was applied to
form Ag/CeO2 plasmonic photocatalysts with multi-
3.5 VLPCR enhancement mechanism
Based on the above results, the mechanism of the enhanced morphologies, in which ve common solvents (H2O, meth-
VLPCR on an Ag–CeO2 composite was proposed, as shown in anol, ethanol, isopropanol, and ethylene glycol) were used.
Fig. 9. In general, the Fermi level (Ef) of CeO2 is higher than that Among these samples, ACM and ACG displayed a high VLPCR
of Ag.33 Hence, when CeO2 and Ag particles were in contact, the activity, achieving a CH4 yield of 89.32 mmol and 99.98 mmol,
electrons on the CeO2 tended to migrate to Ag particles to and CH3OH yield of 32.13 mmol and 34.96 mmol per gram of
achieve the Fermi equilibrium of the system (Fig. 9(a)). In catalyst per hour during a 6 h visible-light irradiation. The
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was nancially supported by the National Natural 23 W. Cai, Q. Zhong, D. Wang, Y. Zhao, M. Chen and Y. Bu, ACS
Science Foundation of China (51808296 and 51802160), the Appl. Nano Mater., 2018, 1, 1150–1163.
Natural Science Foundation of Jiangsu Province of China 24 L. Pastor-Perez, V. Belda-Alcazar, C. Marini, M. M. Pastor-
´
´
´
(BK20170938, BK20150892 and BK20160834), the 63rd Chinese
Postdoctoral Science Foundation (2018M630487), and the
Blas, A. Sepulveda-Escribano and E. V. Ramos-Fernandez,
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Startup Foundation for Introducing Talent of NUIST 25 M. Lykaki, E. Pachatouridou, S. A. C. Carabineiro,
(2243141601034). In addition, Wei Cai would also like to
acknowledge support and helpful discussion from the WECHAT
group named “Energetic and Progressive Mightyboy”!
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